U.S. patent number 6,411,090 [Application Number 09/681,970] was granted by the patent office on 2002-06-25 for magnetic resonance imaging transmit coil.
This patent grant is currently assigned to GE Medical Systems Global Technology Company, LLC. Invention is credited to Eddy B. Boskamp.
United States Patent |
6,411,090 |
Boskamp |
June 25, 2002 |
Magnetic resonance imaging transmit coil
Abstract
A magnetic resonance imaging (MRI) coil system is provided which
comprises an RF coil element comprising a plurality of electrically
conductive members spaced to form a generally tubular structure and
defining an imaging volume. The coil element is adapted to receive
a RF signal and apply a magnetic field to the imaging volume. A
signal generator generates the RF signal, and a power splitter is
adapted to distribute the RF signal across each of the plurality of
electrically conductive members in signals of equal power. A phase
shifter receives the split signals from the signal splitter and
equally phase shifts each of the split RF signals across each of
the plurality of electrically conductive members such that each
conductor carries the signal at a different phase angle with
respect to each other conductor. An amplifier is coupled to each of
the conductors for independently controlling the current amplitude
of the signal carried on the respective conductor.
Inventors: |
Boskamp; Eddy B. (Menomonee
Falls, WI) |
Assignee: |
GE Medical Systems Global
Technology Company, LLC (Waukesah, WI)
|
Family
ID: |
24737635 |
Appl.
No.: |
09/681,970 |
Filed: |
July 2, 2001 |
Current U.S.
Class: |
324/318; 324/309;
324/322 |
Current CPC
Class: |
G01R
33/34046 (20130101); G01R 33/3614 (20130101); G01R
33/3621 (20130101) |
Current International
Class: |
G01R
33/34 (20060101); G01R 33/36 (20060101); G01R
33/32 (20060101); G01V 003/00 () |
Field of
Search: |
;324/318,309,322,307
;128/653 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lefkowitz; Edward
Assistant Examiner: Shrivastav; Brij B.
Attorney, Agent or Firm: Della Penna; Michael A.
Claims
What is claimed is:
1. A radio frequency (RF) coil apparatus for resonance
imaging/analysis comprising:
a plurality of axial conductors spaced to form and generally
tubular structure and define an imaging volume, each axial
conductor having first and second ends;
a first ring conductor coupled to the first ends;
a second ring conductor coupled to the second ends; and
a plurality of amplifiers, each of said amplifiers coupled to a
respective axial conductor for independently controlling a current
amplitude of a signal carried on said respective conductor.
2. The RF coil of claim 1 wherein the plurality of axial conductors
equals between eight and sixteen.
3. The RF coil of claim 1 wherein each amplifier comprises a power
MOSFET amplifier.
4. A magnetic resonance imaging (MRI) coil system comprising:
an RF coil element comprising a plurality of electrically
conductive members spaced to form and generally tubular structure
and defining an imaging volume, said coil element being adapted to
receive a RF signal and apply a magnetic field to said imaging
volume;
a signal generator for generating said RF signal;
a power splitter adapted to distribute said RF signal across each
of said plurality of electrically conductive members;
a phase shifter adapted to phase shift said RF signal across each
of said plurality of electrically conductive members such that each
conductor carries said signal at a different phase angle with
respect to each other conductor; and
a plurality of amplifiers, each of said amplifiers coupled to a
respective conductor for independently controlling a current
amplitude of the signal carried on said respective conductor.
5. The MRI coil system of claim 4 further comprising a controller
adapted to independently control each of said amplifiers.
6. The MRI coil system of claim 5 wherein the RF coil element
comprises sixteen electrically conductive members.
7. The MRI coil system of claim 6 wherein the power splitter is
adapted to split the RF signal into sixteen signals of equal
power.
8. The MRI coil system of claim 7 wherein the phase shifter is
adapted to receive each of said sixteen power split signals from
said power splitter and phase shift each signal by 22.5 degrees
with respect to each other signal.
9. The MRI coil system of claim 4 wherein the RF coil element
comprises eight electrically conductive members.
10. The MRI coil system of claim 6 wherein the power splitter is
adapted to split the RF signal into eight signals of equal
power.
11. The MRI coil system of claim 10 wherein the phase shifter is
adapted to receive each of said eight power split signals from said
power splitter and phase shift each signal by 45 degrees with
respect to each other signal.
12. A phased array coil apparatus for use in an nuclear magnetic
resonance (NMR) system, the coil comprising a plurality of
electrically conductive members spaced to form and generally
tubular structure and defining an imaging volume, said coil element
being adapted to apply a RF magnetic field to said imaging volume,
wherein each of said electrically conductive members is adapted to
receive a RF signal and each of said electrically conductive
members is configured to independently control a current amplitude
of said signal.
13. The phased array coil of said claim 12 wherein each
electrically conductive member includes an amplifier for
independently controlling a current amplitude of said signal.
14. The phased array coil of said claim 13 wherein each of said
amplifiers is a power MOSFET amplifier.
15. The phased array coil of said claim 12 wherein the plurality of
electrically conductive members equals eight.
16.The phased array coil of said claim 12 wherein the plurality of
electrically conductive members equals sixteen.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the field of magnetic resonance
imaging (MRI) systems and, more particularly, concerns radio
frequency (RF) coils for use in such systems.
In MRI systems or nuclear magnetic resonance (NMR) systems, radio
frequency signals are provided in the form of circularly polarized
or rotating magnetic fields having an axis of rotation aligned with
a main magnetic field. An RF field is then applied in the region
being examined in a direction orthogonal to the static field
direction, to excite magnetic resonance in the region, and
resulting RF signals are detected and processed. Receiving coils
intercept the radio frequency magnetic field generated by the
subject under investigation in the presence of the main magnetic
field in order to provide an image of the subject. Typically, such
RF coils are either surface-type coils or volume-type coils,
depending upon the particular application. Normally, separate RF
coils are used for excitation and detection, but the same coil or
array of coils may be used for both purposes.
Conventional MRI systems have a number of artifact problems. For
example, aliasing of unwanted signals into the resonance object
image is a common problem in MRI applications. A particular form of
artifact, sometimes referred to as an aliasing artifact, can occur
in the either the frequency direction or the phase direction within
MRI systems. In this type of artifact, an area of anatomy that is
at least partially within the excitation field of the body coil has
a local Larmor frequency identical to a pixel within the imaging
field of view. This phenomenon typically originates from areas
outside the field of view, but causes artifacts inside the image.
It often arises as a result of the non-linearity of the gradient
fields and/or non-homogeneity of the DC magnetic fields.
Accordingly, to reduce the occurrences of unwanted artifacts, there
exists a need for MRI power can be lost in cables and switches
before the power gets to the transmit coil.
Another advantage of the present invention is that different RF
field shapes can be generated with a single coil thereby enabling
selective excitation within the imaging volume.
Other objects and advantages of the invention will become apparent
upon reading the following detailed description and appended
claims, and upon reference to the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
For a more complete understanding of this invention, reference
should now be made to the embodiments illustrated in greater detail
in the accompanying drawings and described below by way of examples
of the invention.
In the drawings:
FIG. 1 is a planar schematic view of a high pass birdcage coil;
FIG. 2 is a block diagram of a magnetic resonance imaging system
according to one embodiment of the present invention;
FIG. 3 is a schematic diagram of one embodiment of a transmit coil
according to the present invention; and
FIG. 4 is a schematic diagram of one embodiment of connection
circuitry for the transmit coil of FIG. 3.
DETAILED DESCRIPTION
In describing the characteristics of the transmit coil arrangement
of the present invention, it is useful to have an understanding of
distributed-type volume coils in general and a birdcage transmit
coil in particular. In this regard, referring to FIG. 1, there is
shown a planar schematic view of a high pass birdcage coil. A
birdcage coil consists of two rings connected by several straight
segments referred to as legs. A planar schematic of an eight-leg
high-pass birdcage is shown in FIG. 1. This coil consists of two
end rings R1 and R2 and eight legs, 18.
The birdcage which is of the distributed inductance-capacitance
type structure has several frequency modes. Of interest is the
homogeneous mode which sets up a homogeneous RF magnetic field. The
homogeneous mode has two linear components, oriented orthogonal to
one another. At the homogeneous mode, the currents in the coil are
cosinusoidally distributed such that the resultant field displayed
has a homogeneous distribution over the imaging field of view
(FOV). The points where the planes of symmetry intersect the
birdcage are "a, b, c, d, e, f, g, h" on ring R1 and "i, j, k, l,
m, n, o, p" on ring R2, and "q, r, s, t, u, v, w, x" on legs 1, 2,
3, . . . 8, respectively of FIG. 1. Since points q through x are
located on the virtual ground plane, these points are at virtual
ground potential or have no net potential.
Referring now to FIG. 2, there is shown a block diagram of an MRI
system operable to perform a magnetic resonance imaging by using a
radio frequency coil according to the present invention.
The system of FIG. 2 is operated such that a static magnetic field
generating section M may form a uniform static magnetic field in
its inner space. The static magnetic field generating section M is
comprised of a pair of magnetic generators formed by permanent or
superconductive magnets (not shown), for example, these magnetic
generators are spaced apart in a horizontal direction and
oppositely faced to each other so as to form a static magnetic
field (a horizontal magnetic field) in the opposing space. Each of
the gradient coil sections G is arranged at the front surface of
the magnetic generator and they are similarly spaced apart and
opposed in a horizontal direction from each other.
A transmission coil section T forming a cylindrical assembly is
installed within the static magnetic field space between the
gradient coil sections G. A body coil section B forming a
cylindrical assembly is installed within the static magnetic field
space within the transmission coil T. A central axis of the body
coil section B is crossed at a right angle with a direction of the
static magnetic field. Between the gradient coil sections G and the
transmission coil T there is an RF shield S that shields the
transmission coils from the gradient coils. Although separate RF
coils are shown for excitation (transmission coil T) and detection
(body coil B), the same coil or array of coils may be used for both
purposes.
An inspected body O is inserted into the inner space of the body
coil section B. A body axis of the inspected body O is aligned with
a direction of the static magnetic field.
transmission coil section TR is connected to the transmission coil
T. The transmission coil section TR applies a driving signal to the
transmission coil T so as to generate a radio (RF) magnetic field,
thereby a spin in the body of the inspected body O is excited. The
transmission coil T and transmission coil section TR is one example
of a preferred embodiment of the transmission coil of the present
invention. Details of the transmission coil will be described below
with reference to FIG. 3. A gradient driving section GR is
connected to the gradient coil sections G. The gradient driving
section GR applies a driving signal to the gradient coil sections G
so as to generate a gradient magnetic field. To the body coil
section B is connected a receiving section RV. To the receiving
section RV is inputted a magnetic resonance receiving signal
received by the body coil section B.
To the receiving section RV is connected an analog-to-digital
conversion section AD. The analog-to-digital conversion section AD
operates to convert an output signal of the receiving section RV
into a digital signal. The analog-to-digital conversion section AD
is connected to a computer COM. To the computer COM is inputted a
digital signal from the analog-to-digital conversion section AD,
wherein an image reforming process is carried out in response to
the input digital signal, and an image of the inspected body O is
generated.
To the computer COM a displaying section DIS and an operating
section OP are connected. The displaying section DIS displays an
image generated by the computer COM. The displaying section DIS
also displays various kinds of information outputted from the
computer COM. The operating section OP is operated by an operator
so as to input various kinds of instructions or information to the
computer COM.
To the computer COM is also connected a control section CNT. The
control section CNT is connected to the transmission section TR,
the gradient driving section GR, the receiving section RV, the
analog-to-digital conversion section AD and the imaging table on
which the inspected body O rests. The control section CNT receives
instructions from the computer COM and outputs control signals to
each of the transmission section TR, the gradient driving section
GR, the receiving section RV, the analog-to-digital conversion
section AD and the imaging table so as to perform an imaging
operation.
Referring now to FIG. 3 there is shown a schematic block diagram of
one embodiment of the transmit coil T according to the present
invention for use in the exemplary system of FIG. 2. In this
example, the transmit coil comprises a sixteen leg or rung birdcage
30. Thus, between each ring 32, 34 of the birdcage 30 there extends
sixteen legs 36. For simplicity, the birdcage 30 is shown in the
planar configuration. Each leg 36 of the transmit coil 30 includes
an amplifier 38 which is preferably a MOSFET amplifier. Preferably,
each power amplifier 38 comprises a single power MOSFET or a
push-pull configuration amplifier such as MRF154 available from
Motorola Corporation, as well as the associated DC circuitry and
matching circuitry. In this regard, careful attention should be
given to the routing of the DC current so as not to impair the
homogeneity of the DC magnetic field. Preferably, the matching
circuitry is designed for maximum energy transfer between the power
amp 38 and the coil rungs 36, as well as between the different
components of the transmission section TR. In addition, a pickup
loop can be integrated into each leg or rung 36 to monitor the
current amplitude and phase, and the signal can be fed back to the
amplifier to ensure stable levels of amplitude and phase. An
example of such circuitry is discussed below with reference to FIG.
4.
The transmission section TR operates to control the excitation of
the transmission coil 30. In this example, a preamplifier 40
amplifies the RF pulse from the waveform generator 42 to levels
acceptable for the input of the power amplifiers 38 which are
integrated into the transmit coil 30.
In the case of the sixteen-rod birdcage shown in FIG. 3, the
amplified RF pulse is split by a sixteen-way power splitter 44
which divides the power into sixteen equal components. Any
conventional power splitter design can be used for this task and
the implementation thereof be readily apparent to one of skill in
the art of RF electronics. The resulting sixteen power signals 45
are communicated to a sixteen-way phase shifter 46 to provide
sixteen signals 47 having a sinusoidal amplitude distribution. Each
of the signals 47 is then communicated to a corresponding gate of a
power amplifier 38 associated with a birdcage rung 36. In this
example, the phase shift between two neighboring power channels is
22.5.degree. C., which represents 360.degree. C. across channels 1
through 16. In the case of an eight conductor birdcage coil, the
phase shift between two neighboring power channels is 45.degree.
C., which represents 360.degree. C. across the eight conductors.
Other well known components which comprise the birdcage such as
capacitors, inductors, and matching circuits are not shown for the
sake of simplicity.
FIG. 3 is one example of a preferred application according to the
present invention. The illustrated embodiment, however, is intended
to be exemplary and not limiting. Indeed, the integrated power
amplifier configuration of the present invention is equally
applicable to any type of known homogeneous transmit coils
including TEM transmit coils and birdcage coils.
Referring now to FIG. 4 there is shown an exemplary embodiment of
the circuitry connections for a power amplifier and associated rung
of the birdcage coil of FIG. 3. In FIG. 4, the matching network 100
is configured to match the input impendence of the power amplifier
102 (power MOSFET) to the 50 ohm coaxial cable 104 from the phase
shifter. Matching network 106 matches the output impendence of the
power amplifier 102 to the impendence looking into the
corresponding rung 108 of the birdcage coil. In this way, the
energy transfer from the power amplifier 102 to the rung 108 is
maximized. Couplers 110, 112 measure the input signal and output
signal, respectively. Preferably, coupler 112 is in the form of a
pick-up loop to measure the current amplitude in the rung 108. The
respective signals from the input coupler 110 and output coupler
112 are compared at summer 120 after amplification by amplifiers
122, 124. An error signal results if the input and output signals
differ. The resulting error signal is integrated by integrator 126
and communicated to the voltage controlled attenuator 130 which
regulates the input signal until the error is zero.
From the foregoing, it can be seen that there has been brought to
the art a new and improved transmission coil for MRI applications
which provides advantages over conventional transmission coils.
While the invention has been described in connection with one or
more embodiments, it should be understood that the invention is not
limited to those embodiments. On the contrary, the invention covers
all alternatives, modifications, and equivalents, as may be
included within the spirit and scope of the appended claims.
systems having improved linearity of gradient fields and
homogeneity of DC magnetic fields or RF transmit coils with zero
sensitivity outside the field of view.
SUMMARY OF INVENTION
In the present invention, a distributed integrated power amplifier
is proposed which enables the construction of transmit coils having
improved field of view regulation and drop off outside the field of
view.
In particular, the present invention provides a radio frequency
(RF) coil apparatus for resonance imaging/analysis comprising a
plurality of axial conductors spaced to form a generally tubular
structure and define an imaging volume. Each of the axial
conductors has a first end and a second end. A first ring conductor
is coupled to the first ends and a second ring conductor is coupled
to the second ends to form a birdcage coil. An independently
controllable amplifier is coupled to each respective axial
conductor for independently controlling a current amplitude of a
signal carried on said respective conductor.
In another aspect of the invention, a magnetic resonance imaging
(MRI) coil system is provided which comprises an RF coil element
comprising a plurality of electrically conductive members spaced to
form and generally tubular structure and defining an imaging
volume. The coil element is adapted to receive a RF signal and
apply a RF magnetic field to the imaging volume. A signal generator
generates the RF signal, and a power splitter is adapted to
distribute the RF signal across each of the plurality of
electrically conductive members in signals of equal power. A phase
shifter receives the split signals from the signal splitter and
equally phase shifts each of the split RF signals across each of
the plurality of electrically conductive members such that each
conductor carries the signal at a different phase angle with
respect to each other conductor. An amplifier is coupled to each of
the conductors for independently controlling the current amplitude
of the signal carried on said respective conductor.
An advantage of the present invention is that it enables higher
order transmit coils and arrays which eliminate artifacts by not
exciting the artifact regions. Another advantage is that the
present invention provides precise current amplitude and phase
control throughout the transmit coil, thereby ensuring optimal
homogeneity of the RF magnetic field.
A further advantage of the present invention is that it eliminates
power losses between the power amp and the RF coil, in contrast to
typical MRI systems wherein between 1 and 2 dB of
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